Inhibitors of human immunodeficiency virus (HIV) protease have been approved for use in the treatment of HIV infection for several years. A particularly effective HIV protease inhibitor was (5S,8S,10S,11S)-10-Hydroxy-2-methyl-5-(1-methylethyl)-1-[2-(1-methylethyl)-4-thiazolyl]-3,6-dioxo-8,11-bis(phenylmethyl)-2,4,7,12-tetraazatridecan-13-oic acid 5-thiazolylmethyl ester, also known as Ritonavir.

Ritonavir and its process were disclosed in U.S. Pat. No. 5,541,206.
Polymorphism is defined as “the ability of a substance to exist as two or more crystalline phases that have different arrangement and/or conformations of the molecules in the crystal Lattice. Thus, in the strict sense, polymorphs are different crystalline structures of the same pure substance in which the molecules have different arrangements and/or different configurations of the molecules”. Different polymorphs may differ in their physical properties such as melting point, solubility, X-ray diffraction patterns, etc. Although those differences disappear once the compound is dissolved, they can appreciably influence pharmaceutically relevant properties of the solid form, such as handling properties, dissolution rate and stability. Such properties can significantly influence the processing, shelf life, and commercial acceptance of a polymorph. It is therefore important to investigate all solid forms of a drug, including all polymorphic forms, and to determine the stability, dissolution and flow properties of each polymorphic form. Polymorphic forms of a compound can be distinguished in the laboratory by analytical methods such as X-ray diffraction (XRD), Differential Scanning calorimetry (DSC) and Infrared spectrometry (IR).
Solvent medium and mode of crystallization play very important role in obtaining one polymorphic Form over the other.
Ritonavir can exist in different polymorphic Forms, which may differ from each other in terms of stability, physical properties, spectral data and methods of preparation.
Crystalline Form II of ritonavir was disclosed in U.S. Pat. No. 6,894,171. According to the patent also described crystalline form I of ritonavir.
U.S. Pat. No. 7,205,413 disclosed crystalline Form III, Form IV and Form V of ritonavir.
U.S. Pat. No. 7,148,359 disclosed a substantially pure amorphous ritonavir.
Process for the preparation of substantially pure amorphous ritonavir was disclosed in U.S. Pat. No. 7,183,416. According to the patent, substantially pure amorphous ritonavir can be prepared by adding a solution of ritonavir containing methanol or methylene chloride to an anti-solvent such as hexane or methyl t-butyl ether and isolating.
Process for the preparation of amorphous ritonavir was described in Journal of Pharmaceutical Sciences, Vol. 90, No. 8, P. 1015-1025 by heating the ritonavir to 135° C. in oil bath, followed by rapid cooling using liquid nitrogen or cold water.
Process for the preparation of amorphous ritonavir was described in Journal of Pharmaceutical Sciences, Vol. 91, No. 8, P. 1863-1872 by freeze-drying.
U.S. Pat. No. 5,559,158 disclosed a solid pharmaceutical composition of ritonavir having the composition is encapsulated in a hard gelatin capsule.
U.S. Pat. No. 5,948,436 disclosed pharmaceutical composition comprising a solution of ritonavir having the solution is encapsulated in a hard gelatin capsule or a soft elastic gelatin capsule.
U.S. Pat. No. 7,364,752 disclosed compositions of ritonavir prepared by solid dispersion technique.
Preparation of amorphous Form of ritonavir was described in Journal of the American association of pharmaceutical scientists, Vol. 13, No. 9, P. 7473-7476, September 1996 by spray drying of the ritonavir on polyvinylpyrillodone (PVP) and ethanol. Similarly, preparation of amorphous Form of ritonavir was described by layering of the ritonavir on microcrystalline cellulose and silicon dioxide.
It was observed that the amorphous material obtained by the process described in U.S. Pat. No. 7,181,416 was changed into molten state within about 30 minutes. Thus, the amorphous form obtained by the process of U.S. Pat. No. 7,181,416 was shown to be unstable.
We have found a novel amorphous ritonavir co-precipitated on copovidone. The amorphous ritonavir co-precipitated on copovidone is stable, reproducible and so, the amorphous ritonavir co-precipitated on copovidone is suitable for formulating ritonavir. Normally amorphous Forms are hygroscopic. Amorphous ritonavir co-precipitated on copovidone is found to be non-hygroscopic.
Thus, an object of the present invention is to provide amorphous ritonavir co-precipitated on copovidone, process for its preparation and pharmaceutical compositions comprising it.